Light in the near- to mid-infrared region corresponds to an absorption spectrum region related to biological objects such as plants and animals and the environment. Therefore, the development of a photodetector in the near- to mid-infrared region, the photodetector including a light-receiving layer composed of a group III-V compound semiconductor, has been conducted. A known example of a detector that converts photo-induced charge into an output signal includes a light receiving element array in which the responsivity is extended to a wavelength of 2.6 μm by using extended-InGaAs as a light-receiving layer, and a complementary metal-oxide semiconductor (CMOS) functioning as a read-out integrated circuit (ROIC) is connected to the light receiving element array (NPL 1). The light receiving element array includes a window layer composed of InAsP that is lattice-matched with the InGaAs light-receiving layer.
It has also been reported that a p-i-n-type photodiode including a light-receiving layer having a type-II multiple quantum well structure (MQW) composed of InGaAs/GaAsSb and a p-type pixel region has a responsivity up to a wavelength of 2.5 μm (NPL 2).
A light receiving element including a GaSb substrate and a light-receiving layer provided on the GaSb substrate, the light-receiving layer having a type-II MQW composed of (InAs/GaSb) has also been proposed (NPL 3). It is described that this light receiving element has a responsivity up to a wavelength of close to 12 μm.
A light receiving element having an n-B-n (n-type layer/barrier layer/n-type layer) structure in which a light-receiving layer having a type-II MQW composed of (InAs/GaSb) is provided on a GaSb substrate and a barrier layer is arranged in the middle of the light-receiving layer has also been proposed (NPL 4). As compared with the p-i-n structure, the n-B-n structure is advantageous in that, since diffusion of a hole is used for detecting light, the depth of mesa etching for pixel isolation can be made small and a noise current flowing in a side wall of a mesa structure can be reduced.